In-channel memory mirroring

ABSTRACT

Embodiments of the present invention provide methods, program products, and systems for improving DIMM level memory mirroring. Embodiments of the present invention can be used to configure a first memory module device of a pair memory module devices to receive a set of read and write operations and configure a second memory module device of the pair of memory module devices to receive only write operations of the set of read and write operations. Embodiments of the present invention can, responsive to detecting a failure, reconfiguring the first and the second memory module device to set the first memory module device to receive only write operations of the set of read and write operations and the second memory module device to receive read and write operations of the set of read and write operations.

BACKGROUND

The present invention relates generally to the field of memory modules,and more particularly to memory module devices.

Computer memory generally refers to any physical device that is capableof storing information temporarily or permanently. Typically, memory caneither be volatile (i.e., loses its content when the device loses power)or non-volatile (i.e., retains its contents even if power is lost).Examples of volatile memory include memory module devices such as SingleIn-Line Memory Modules (SIMM) and Dual In-Line Memory Modules (DIMM). Incases of unexpected power loss, data residing in either volatile memorymodule device are lost and cannot be recovered. In DIMM level memorymirroring, data from one DIMM is mirrored to another DIMM to providedata redundancy. IF any one DIMM fails with unrecoverable error, systemoperations can continue using the other DIMM.

SUMMARY

In one embodiment of the present invention, a computer-implementedmethod is providing comprising: configuring a first memory module deviceof a pair memory module devices to receive a set of read and writeoperations; configuring a second memory module device of the pair ofmemory module devices to receive only write operations of the set ofread and write operations; and responsive to detecting a failure,reconfiguring the first and the second memory module device to set thefirst memory module device to receive only write operations of the setof read and write operations and the second memory module device toreceive read and write operations of the set of read and writeoperations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a computing environment, in accordance withan embodiment of the present invention;

FIG. 2 is a flowchart illustrating operational steps for improving DIMMlevel memory mirroring, in accordance with an embodiment of the presentinvention;

FIG. 3 is a flowchart illustrating operational steps for reconfiguring apair of DIMMS, in accordance with an embodiment of the presentinvention;

FIG. 4 is an example diagram that is helpful in understanding hardwareset up that enables memory mirroring, in accordance with an embodimentof the present invention; and

FIG. 5 is a block diagram of internal and external components of thecomputer systems of FIG. 1, in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION

Embodiments of the present invention recognize that failed DIMMs(Dual-Inline Memory Modules) can cause service disruptions in customerenvironments. In some instances, failed DIMMs can be replaced at thecost of service disruption. In other instances, a dual drop DIMMconfiguration (e.g., where mirroring a first DIMM to a second DIMM) canconsume two times the write bandwidth as write data is sent to bothmirrored and mirroring copies). Embodiments of the present inventionprovide solutions to improve the reliability, availability, and serviceof DIMM level memory mirroring. In this manner, as discussed in greaterdetail later in this specification, embodiments of the present inventioncan improve DIMM level memory mirroring by configuring a pass-thru mode(e.g., allowing all bus transaction) on a first DIMM and a configuring asecond DIMM to a mirror mode (e.g., where it allows only writes toDRAMS). In the event of the first DIMM experiencing a failure,embodiments of the present invention can enable memory mirroring on thesame channel and reconfigure the second DIMM to the pass-thru mode toallow all bus transactions which allows users to resume operations withminimal or no down time.

FIG. 1 is a functional block diagram of computing environment 100, inaccordance with an embodiment of the present invention. Computingenvironment 100 includes processor 102. Processor 102 can be a desktopcomputer, laptop computer, specialized computer servers, or any othercomputer system known in the art. In certain embodiments, processor 102represents computer systems utilizing clustered computers and componentsto act as a single pool of seamless resources when accessed through anetwork. For example, such embodiments may be used in data center, cloudcomputing, storage area network (SAN), and network attached storage(NAS) applications. In certain embodiments, processor 102 represents avirtual machine. In general, processor 102 is representative of anyelectronic device, or combination of electronic devices, capable ofexecuting machine-readable program instructions, as described in greaterdetail with regard to FIG. 4.

Processor 102 includes memory controller unit 104 (MCU 104). MCU 104facilitates the flow of data going to and from memory associated withprocessor 102 (e.g., memory module device pair 110). For example, MCU104 can configure a pair of memory module devices (e.g., a pair ofDIMMs) to have two modes so as to allow users to resume operations withminimal or no down time in the event of a failure, as discussed ingreater detail later with regard to FIGS. 2 and 3. A “failure” as usedherein, can be any abnormal termination, interruption, or error insoftware and/or hardware in processor 102 that affects access toprocessor 102 or another component of computing environment 100. Forexample, a failure can be an unrecoverable error, that is, an error thatoccurs during a computer's (e.g., processor 102) operation of a code orprogram that has not been registered before and no amount retries cancorrect or undo the error.

In this embodiment, MCU 104 can configure memory module devices tofunction in a pass thru or mirror mode. A “pass thru mode” as usedherein, refers to a mode wherein a memory module device (e.g., a firstDIMM of a pair of DIMMs) is configured to allow all bus transactions,for example, read and write operations. A “mirror mode” as used herein,refers to a mode wherein a memory module device (e.g., a second DIMM ofa pair of DIMMs) is configured to allow only write operations to DynamicRandom Access Memory (DRAMs).

In this embodiment, responsive to detecting a failure, MCU 104 canreconfigure the pair of DIMMs so as to allow users to resume operationswith minimal or no down time by enabling in-channel memory mirroring. A“failure” as used herein, refers generally to any abnormal termination,interruption, or error in software and/or hardware in the processoranother component of the computing environment that affects processor102 (e.g., loss of power via a drop in voltage). For example, the firstDIMM of a pair of DIMMs configured to function in the pass thru modewhile the second DIMM can be configured to function in the mirror mode.Responsive to detecting that a failure occurred on the first DIMM, MCU104 can reconfigure the second DIMM to receive read and writeoperations. In other words, MCU 104 can reconfigure the second DIMM tofunction in the pass thru mode. MCU 104 can then reconfigure the firstDIMM to function in mirror mode. Accordingly, operations can resume withlittle or no down time.

In this embodiment, MCU 104 is connected to memory buffering unit 106(MBU 106). MCU 104 can leverage MBU 106 to temporarily store data froman input device (e.g., processor 102) to an output device (e.g., memorymodule device pair). MBU 106 is also capable of performing variousmemory controller operations as well (e.g., schedule memory operationsbased on requests received from MCU 104 such as read re-order, writecommand re-order, etc.). MBU 106 includes on-demand memory mirroringunit 108 (OMM 108). OMM 108 interacts with memory module device pair 110performs on demand memory mirroring to DIMMs of memory module devicepair 110. OMM 108 controls and can recognize operational modesassociated with memory module devices (e.g., pass thru and mirrormodes). OMM 108 also controls DRAM MRS register configurations.

Memory module device pair 110 are a pair of memory module devices. A“memory module device” as used herein, refers generally to a hardwarestorage device having one or more memory modules. For example, a memorymodule device can be Single In-Line Memory Module (SIMM), a Dual In-LineMemory Module (DIMM), and/or other types of hardware storage deviceshaving one or more memory modules, as will be appreciated by those ofordinary skill in the art.

In this embodiment, memory module device pair 110 are a pair of DIMMsthat include a first and second DIMM. Specifically, memory module devicepair 110 includes near end DIMM (e.g., NED 112) and far end DIMM (e.g.,FED 114). In this embodiment, NED 112 and FED 114 are depicted as havingnon-shared, independent connections to MBU 106. In other embodiments,NED 112 and FED 114 can have a bifurcated connection. In yet otherembodiments, NED 112 and FED 114 can be connected in series.

It should be understood that, for illustrative purposes, FIG. 1 does notshow other computer systems and elements which may be present whenimplementing embodiments of the present invention. For example, whileFIG. 1 shows a single processor (e.g., processor 102), a single MCU(e.g., MCU 104), and a pair of DIMMs (e.g., memory module device pair110). However, computing environment can include multiple processors,MCUs, and DIMM pairs.

FIG. 2 is a flowchart 200 illustrating operational steps for improvingDIMM level memory mirroring, in accordance with an embodiment of thepresent invention.

In step 202, MCU 104 transmits an instruction to MBU 106 to perform aninitial calibration to calculate delay. In this embodiment, MCU 104transmits an instruction to MBU 106 to perform an initial calibration tocalculate delay when the system is initially powered on. MBU 106calculates the exact delay required to perform a read and writeoperation to a particular memory module device (e.g., NED 112 and FED114). For example, MBU 106 calculates the exact delay to find out theboard trace length, memory buffer IO delay, and traditional memory usedto write data on the memory module devices. The calibration is needed toestablish memory controller and gate settings to ensure proper write andread operations to and from DRAM memory. In other words, MBU 106 canperform an initial calibration to calculate and vary delays iterativelyuntil a point where maximum read and write timing margins are ensured atbyte lane granularity to account for length variation among byte lanes.MBU 106 can then store the calculated delay for future transactions. MBU106 can then update the calculated delay based on process/voltage andtemperature variations.

In step 204, MCU 104 configures the first DIMM (e.g., NED 112) to passthru mode. In this embodiment, MCU 104 configures the first DIMM to passthru mode by transmitting instructions to the first DIMM via OMM 108 toallow only write operations and to share address line space between thefirst and second DIMM. In other words, only write operations areperformed on the second DIMM.

In step 206, MCU 104 configures the second DIMM (e.g., FED 114) tomirror mode. In this embodiment, MCU 104 configures the second DIMM bytransmitting instructions to the first DIMM via OMM 108 to allow writeoperations on FED 114. Accordingly, OMM 108 can mirror data of the firstDIMM to the second DIMM using the shared address space between the firstand the second DIMM. For example, OMM 108 can get 128 byte cache linefrom the first DIMM (e.g., NED 112) and mirror it to the second DIMM(e.g., FED. 112).

In step 208, MCU 104 reconfigures the first and the second DIMMresponsive to detecting a failure. In this embodiment, MCU 104reconfigures the first and the second DIMM by sharing signals (e.g., viaa set of multiplexers initiated by OMM 108 in response to receivinginstructions from MCU 104) to achieve chip selection (i.e., selectingdifferent memory module devices) to both the DIMMS (e.g., NED 112 andFED 114), and other signals such as on-die termination (ODT), clockenable (CKE), address lines, etc. to both DIMMs, as discussed in greaterdetail with regard to FIG. 3. Accordingly writes are allowed to both theDIMMs.

FIG. 3 is a flowchart 300 illustrating operational steps forreconfiguring a pair of DIMMS, in accordance with an embodiment of thepresent invention.

In step 302, MCU 104 configures the second DIMM to function in pass thrumode. In this embodiment, MCU 104 configures the second DIMM to functionin pass thru mode by sharing signals such as CS to both DIMMs. Forexample, MCU 104 can transmit an instruction to switch the CS line tobehave in the pass thru mode. As mentioned before a “pass thru mode” asused herein, refers to a mode wherein a memory module device (e.g., afirst DIMM of a pair of DIMMs) is configured to allow all bustransactions, for example, read and write operations.

In step 304, MCU 104 configures the first DIMM to function in mirrormode. In this embodiment, MCU 104 configures the second DIMM to functionin pass thru mode by transmitting instructions to OMM 108 to sharesignals such as CS to both DIMMs (e.g., CS0, and CS1). For example, MCU104 can transmit an instruction to switch the CS line to behave the waywhen mirror is enabled. Accordingly, the CS0 and 1 will transmitoperations to FED 114 and CS2 will transmit operations to NED 112. Asmentioned before, a “mirror mode” as used herein, refers to a modewherein a memory module device (e.g., a second DIMM of a pair of DIMMs)is configured to allow only write operations to Dynamic Random AccessMemory (DRAMs).

Accordingly, new read and write operations can be performed on thesecond DIMM while write operations can be performed on the first DIMM.For example, MCU 104 can continue to perform write operations to thefirst DIMM to mirror the write operations performed on the second DIMM.In this embodiment, MCU 104 can continue to perform write operations tothe first DIMM by identifying a specific location associated with thepoint of failure and resume write operations from the point of failure.

FIG. 4 is an example diagram 400 that is helpful in understandinghardware set up that enables memory mirroring, in accordance with anembodiment of the present invention.

In this example, diagram 400 represents a computing environment presentsan alternate configuration of components described in FIG. 1. Forexample, processor 402, memory controller unit (MCU) MCU 404, on-demandmemory mirroring unit (OMM) OMM 108 and memory buffering unit (MBU) MBU408, and MBU 412 are counterparts to processor 102, MCU 104, OMM 108 andMBU 106.

Diagram 400 also includes a pair of DIMM 450 and DIMM 460. In thisexample, DIMM 450 serves as the DIMM that is configured in a “pass thrumode”. As mentioned before a “pass thru mode” as used herein, refers toa mode wherein a memory module device (e.g., a first DIMM of a pair ofDIMMs) is configured to allow all bus transactions, for example, readand write operations. DIMM 460 serves as the DIMM configured in a“mirror mode” which refers to a mode wherein a memory module device(e.g., a second DIMM of a pair of DIMMs) is configured to allow onlywrite operations to Dynamic Random Access Memory (DRAMs). Each of DIMM450 and DIMM 460 includes a respective MBU unit (e.g., MBU 408 and MBU408′) and respective DRAM units (e.g., DRAM 410A-N of DIMM 450 and DRAM420A-N of DIMM 460.

In this example, MCU 404 has detected a failure with DIMM 450. MCU 404has transmitted a command to OMM 406 to initiate the reconfigurationprocess and OMM 406 has reconfigured DIMM 460 to a pass thru mode andDIMM 450 to a mirror mode via an inline bus. Accordingly, new read andwrite operations can be performed on DIMM 460 while “write operations”can be performed from the point of failure identified by MCU 404. MCU404 can then continue to perform updated write operations on torespective DRAMS of DIMM 450 to mirror the writes performed on DIMM 460.

Embodiments of the present invention recognize problems with dual dropconfiguration which consume two times write bandwidth as write dataneeds to be sent to both mirrored and mirroring copies. However,embodiments of the present invention provide solutions to minimize thewrite bandwidth by providing memory buffering units inside the DRAM withan ability to register, regenerate, and filter specific transactions tosupport mirror of data in another DRAM which is achieved with the helpof the in line bus between memory buffering units of DRAMS whichpreserves memory bandwidth and read transactions from the mirrored DRAM.Embodiments of the present invention further provide capabilities toconfigure DRAMs in either pass thru or mirror mode using mode, register,set (MRS) commands from a memory controller unit. Specifically,embodiments of the present invention further provide capabilities toswitch between pass thru or mirror modes in response to detecting afailure.

FIG. 5 is a block diagram of internal and external components of acomputer system 500, which is representative the computer systems ofFIG. 1, in accordance with an embodiment of the present invention. Itshould be appreciated that FIG. 5 provides only an illustration of oneimplementation and does not imply any limitations with regard to theenvironments in which different embodiments may be implemented. Ingeneral, the components illustrated in FIG. 5 are representative of anyelectronic device capable of executing machine-readable programinstructions. Examples of computer systems, environments, and/orconfigurations that may be represented by the components illustrated inFIG. 5 include, but are not limited to, personal computer systems,server computer systems, thin clients, thick clients, laptop computersystems, tablet computer systems, cellular telephones (e.g., smartphones), multiprocessor systems, microprocessor-based systems, networkPCs, minicomputer systems, mainframe computer systems, and distributedcloud computing environments that include any of the above systems ordevices.

Computer system 500 includes communications fabric 502, which providesfor communications between one or more processors 504, memory 506,persistent storage 508, communications unit 512, and one or moreinput/output (I/O) interfaces 514. Communications fabric 502 can beimplemented with any architecture designed for passing data and/orcontrol information between processors (such as microprocessors,communications and network processors, etc.), system memory, peripheraldevices, and any other hardware components within a system. For example,communications fabric 502 can be implemented with one or more buses.

Memory 506 and persistent storage 508 are computer-readable storagemedia. In this embodiment, memory 506 includes random access memory(RAM) 516 and cache memory 518. In general, memory 506 can include anysuitable volatile or non-volatile computer-readable storage media.Software is stored in persistent storage 508 for execution and/or accessby one or more of the respective processors 504 via one or more memoriesof memory 506.

Persistent storage 508 may include, for example, a plurality of magnetichard disk drives. Alternatively, or in addition to magnetic hard diskdrives, persistent storage 508 can include one or more solid state harddrives, semiconductor storage devices, read-only memories (ROM),erasable programmable read-only memories (EPROM), flash memories, or anyother computer-readable storage media that is capable of storing programinstructions or digital information.

The media used by persistent storage 508 can also be removable. Forexample, a removable hard drive can be used for persistent storage 508.Other examples include optical and magnetic disks, thumb drives, andsmart cards that are inserted into a drive for transfer onto anothercomputer-readable storage medium that is also part of persistent storage508.

Communications unit 512 provides for communications with other computersystems or devices via a network. In this exemplary embodiment,communications unit 512 includes network adapters or interfaces such asa TCP/IP adapter cards, wireless Wi-Fi interface cards, or 3G or 4Gwireless interface cards or other wired or wireless communication links.The network can comprise, for example, copper wires, optical fibers,wireless transmission, routers, firewalls, switches, gateway computersand/or edge servers. Software and data used to practice embodiments ofthe present invention can be downloaded through communications unit 512(e.g., via the Internet, a local area network or other wide areanetwork). From communications unit 512, the software and data can beloaded onto persistent storage 508.

One or more I/O interfaces 514 allow for input and output of data withother devices that may be connected to computer system 500. For example,I/O interface 514 can provide a connection to one or more externaldevices 520 such as a keyboard, computer mouse, touch screen, virtualkeyboard, touch pad, pointing device, or other human interface devices.External devices 520 can also include portable computer-readable storagemedia such as, for example, thumb drives, portable optical or magneticdisks, and memory cards. I/O interface 514 also connects to display 522.

Display 522 provides a mechanism to display data to a user and can be,for example, a computer monitor. Display 522 can also be an incorporateddisplay and may function as a touch screen, such as a built-in displayof a tablet computer.

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the invention.The terminology used herein was chosen to best explain the principles ofthe embodiment, the practical application or technical improvement overtechnologies found in the marketplace, or to enable others of ordinaryskill in the art to understand the embodiments disclosed herein.

What is claimed is:
 1. A computer-implemented method comprising:responsive to detecting a failure, reconfiguring a first and a secondmemory module device to set the first memory module device to receiveonly write operations of a set of read and write operations and thesecond memory module device to receive read and write operations of theset of read and write operations, wherein reconfiguring the first andthe second memory module devices comprises: transmitting a set ofinstructions to an on demand memory mirroring unit to share addressspace between the first and the second memory module device.
 2. Thecomputer-implemented method of claim 1, further comprising: identifyinga failed location of the first memory module device; reading data of thefirst memory module device at the identified failed location of thefirst memory module device; and performing read and write operations onthe second memory module device from the identified failed location ofthe first memory module device.
 3. The computer implemented method ofclaim 2, further comprising: updating the first memory module devicewith data written to the second memory module device.
 4. Thecomputer-implemented method of claim 1, wherein the first and the secondmemory module device are Dual In-Line Memory Modules.
 5. The computerimplemented method of claim 1, wherein the set of instructions to shareaddress space between the first and the second memory module devicecomprises: on-die termination (ODT), clock enable (CKE), and addresslines.
 6. The computer-implemented method of claim 1, wherein detectinga failure comprises: identifying a loss of power to the first and thesecond memory module devices.
 7. The computer-implemented method ofclaim 1, wherein prior to a failure, the first memory module device of apair memory module devices is configured to receive a set of read andwrite operations, and the second memory module device of the pair ofmemory module devices is configured to receive only write operations ofthe set of read and write operations.
 8. A computer program productcomprising: one or more computer readable storage media and programinstructions stored on the one or more computer readable storage media,the program instructions comprising: program instructions to, responsiveto detecting a failure, reconfigure a first and a second memory moduledevice to set the first memory module device to receive only writeoperations of a set of read and write operations and the second memorymodule device to receive read and write operations of the set of readand write operations, wherein the program instructions to reconfigurethe first and the second memory module devices comprise: programinstructions to transmit a set of instructions to an on demand memorymirroring unit to share address space between the first and the secondmemory module device.
 9. The computer program product 8, wherein theprogram instructions stored on the one or more computer readable storagemedia further comprise: program instructions to identify a failedlocation of the first memory module device; program instructions to readdata of the first memory module device at the identified failed locationof the first memory module device; and program instructions to performread and write operations on the second memory module device from theidentified failed location of the first memory module device.
 10. Thecomputer program product of claim 9, wherein the program instructionsstored on the one or more computer readable storage media furthercomprise: program instructions to update the first memory module devicewith data written to the second memory module device.
 11. The computerprogram product of claim 8, wherein the first and the second memorymodule device are Dual In-Line Memory Modules.
 12. The computer programproduct of claim 8, wherein the set of instructions to share addressspace between the first and the second memory module device comprises:on-die termination (ODT), clock enable (CKE), and address lines.
 13. Thecomputer program product of claim 8, wherein the program instructions todetect a failure comprise: program instructions to identify a loss ofpower to the first and the second memory module devices.
 14. Thecomputer program product of claim 8, wherein prior to a failure, thefirst memory module device of a pair memory module devices is configuredto receive a set of read and write operations, and the second memorymodule device of the pair of memory module devices is configured toreceive only write operations of the set of read and write operations.15. A computer system comprising: one or more computer processors; oneor more computer readable storage media; and program instructions storedon the one or more computer readable storage media for execution by atleast one of the one or more computer processors, the programinstructions comprising: program instructions to, responsive todetecting a failure, reconfigure a first and a second memory moduledevice to set the first memory module device to receive only writeoperations of a set of read and write operations and the second memorymodule device to receive read and write operations of the set of readand write operations, wherein the program instructions to reconfigurethe first and the second memory module devices comprise: programinstructions to transmit a set of instructions to an on demand memorymirroring unit to share address space between the first and the secondmemory module device.
 16. The computer system of claim 15, wherein theprogram instructions stored on the one or more computer readable storagemedia further comprise: program instructions to identify a failedlocation of the first memory module device; program instructions to readdata of the first memory module device at the identified failed locationof the first memory module device; and program instructions to performread and write operations on the second memory module device from theidentified failed location of the first memory module device.
 17. Thecomputer system of claim 16, wherein the program instructions stored onthe one or more computer readable storage media further comprise:program instructions to update the first memory module device with datawritten to the second memory module device.
 18. The computer system ofclaim 15, wherein the first and the second memory module device are DualIn-Line Memory Modules.
 19. The computer system of claim 15, wherein theset of instructions to share address space between the first and thesecond memory module device comprises: on-die termination (ODT), clockenable (CKE), and address lines.
 20. The computer system of claim 15,wherein the program instructions to detect a failure comprise: programinstructions to identify a loss of power to the first and the secondmemory module devices.